Download DN290 - Monitor Network Compliant -48V Power Supplies

advertisement
Monitor Network Compliant – 48V Power Supplies – Design Note 290
Brendan Whelan
Introduction
Reliability is a top priority for the designers of modern
telephone and communication equipment. Designers take
extra care to protect circuitry from failure-causing temperature and voltage changes, employing redundancy
whenever possible, especially for power supplies. The
power supplies are monitored to obtain early warning of
impending failure. Often complicated circuitry that can
include a voltage reference, comparators, voltage regulator and several precision resistor dividers is used. Designers may also use discrete components to monitor and
indicate the state of power supply fuses. The resulting
circuitry can be expensive in terms of component cost,
board space and engineering time. The LTC1921 replaces
the complicated monitoring circuitry with a simple integrated precision monitoring system contained entirely in
an 8-lead MSOP package.
Features
The LTC1921 is the only integrated solution that can
monitor two independent – 48V power supplies, plus
associated fuses and drive up to three optoisolators or
LEDs to indicate status. The required external components are three resistors and optocouplers or LEDs, as
shown by the simple circuit in Figure 1. The LTC1921 can
withstand ±100V DC at the supply and fuse input pins and
tolerates ±200V transients.
The LTC1921 monitors supply voltages by dividing the
voltage internally and comparing to an internal precision
reference. Since no critical precision external components are required, component cost, board space and
design time are minimized while accuracy is maximized.
The LTC1921 comes with telecom industry accepted
preset voltage thresholds, including undervoltage
(–38.5V), undervoltage recovery (–43V) and overvoltage
(–70V). The overvoltage threshold has a 1.3V hysteresis
that defines the overvoltage recovery threshold. These
thresholds are trimmed to meet exacting requirements.
This eliminates the messy worst-case threshold tolerance error calculation required when using discrete comparators, resistors and a separate voltage reference.
, LTC and LT and are registered trademarks of Linear Technology Corporation.
Hot Swap is a trademark of Linear Technology Corporation.
47k
5V
FUSE
STATUS
–48V
RETURN
R1
100k
MS8
U A L SIZ
E
A
CT
R2
100k
MOC207
3
RTN
1
8
OUT F
VA
5V
SUPPLY A
STATUS
VB
LTC1921
2
47k
4
FUSE B
OUT A
OUT B
SUPPLY A
–48V
SUPPLY B
–48V
F1
D1
F2
D2
5V
SUPPLY B
STATUS
5
6
DN290 F01
MOC207
R3
47k
1/4W
SUPPLY A
STATUS
0
0
1
1
SUPPLY B
STATUS
0
1
0
1
OK: WITHIN SPECIFICATION
OV: OVERVOLTAGE
UV: UNDERVOLTAGE
MOC207
FUSE A
47k
7
VA
VB
OK
OK
OK
UV OR OV
UV OR OV
OK
UV OR OV UV OR OV
–48V OUT
FUSE A
= VA
= VA
≠ VA
≠ VA
FUSE B
= VB
≠ VB
= VB
≠ VB
FUSE STATUS
0
1
1
1*
0: LED/PHOTODIODE ON
1: LED/PHOTODIODE OFF
*IF BOTH FUSES (F1 AND F2) ARE OPEN,
ALL STATUS OUTPUTS WILL BE HIGH
SINCE R3 WILL NOT BE POWERED
= LOGIC COMMON
Figure 1. The LTC1921 Requires Few External Components for Monitoring Two Supplies
07/02/290
www.BDTIC.com/Linear
The LTC1921 is designed to indicate proper supply status
across a wide variety of conditions. In order to accomplish
this, the internal architecture is symmetrical. The LTC1921
is powered via the supply monitor input pins, VA and VB.
Supply current can be drawn from either or both pins so
the device can operate properly as long as one supply is
within the operating range. Since power is not drawn from
a combined supply (such as would be available with a
diode OR), the LTC1921 will function properly even if the
fuses or diodes are not functional. In addition, the LTC1921
has a low voltage lockout. If both supply voltages are very
low, all three outputs of the LTC1921 lock into a fault
indication state, thus communicating to supervisory systems that there is a problem even though there is not
enough power for the LTC1921 to maintain accuracy.
The LTC1921 can communicate supply and fuse status by
controlling external optocouplers or LEDs. This allows for
intelligent system monitoring despite high isolation voltage requirements. Control of the LEDs or optocouplers is
accomplished by connecting the LTC1921 outputs in
parallel with the LEDs or photodiodes. During normal
supply and fuse conditions, the LTC1921 outputs are high
impedance; current flows through the external diodes
continuously. If a fuse opens or a supply voltage falls
outside of the allowed window, then the proper LTC1921
output shunts the current around the diode, thus indicating a fault. The outputs may be ORed to reduce the number
of required optoisolators.
Application Example
Figure 2 shows an LTC1921 and an LT®4250 Hot SwapTM
controller comprising a complete power system solution.
The LTC1921 monitors both –48V supply inputs from the
power bus, as well as the supply fuses. The status signals
may be wired off the board via optoisolators to an isolated
microprocessor or microcontroller to control system
performance and warning functions. Resistors R9 and
R10 pull up the fuse pins so that damaged fuses can be
detected. The LT4250L controls the combined –48V
supply during hot swapping and low supply conditions,
and monitors the combined supply voltage. The PWRGD
pin drives an optoisolator signalling the status of the
LT4250’s switched output.
The device monitors fuses by comparing the voltage
potentials on each side of each fuse. If a significant
difference (about 2V) is sensed, the LTC1921 signals that
a fuse has opened. The voltage difference across the
damaged fuse may be reduced by diode reverse leakage,
making it difficult to detect a damaged fuse. Weak pull-up
resistors (R1 and R2 in Figure 1) ensure that the LTC1921
can detect an open fuse circuit. The value of the pull-up
resistors used is a function of the reverse leakage current
of the OR’ing diodes used.
– 48V
RTN
R9
10k
1W
R10
10k
1W
MOC207
3
RTN
1
8
VA
OUT F
7
C8
100nF
100V
MOC207
SUPPLY A
STATUS
R4
549k
1%
FUSE A
OUT A
OUT B
3A
– 48V A
R7
51k
5%
8
VDD
VB
FUSE B
R8
100Ω
4
LTC1921
2
FUSE
STATUS
R5
6.49k
1%
5
6
MOC207
R11
47k
1/4W
3A
SUPPLY B
STATUS
PWRGD
3
2
R6
10k
1%
LT4250L
OV
GATE
VEE
*
* DIODES INC. SMAT70A
– 48V B
DRAIN
UV
7
6
+
C2
15nF
100V
SENSE
5
4
R1
0.02Ω
5%
LUCENT
JW050A1-E
MOC207
1
C1
470nF
25V
VIN
R3
1k
5%
R2
10Ω
5%
C3
0.1µF
100V
1N4003
VOUT
1
LUCENT
FLTR100V10
VIN–
2
+
C4
0.1µF
100V
+
C5
100µF
100V
VOUT–
CASE
C6
0.1µF
100V
4
VIN+
VOUT+
SENSE +
TRIM
ON/OFF
SENSE –
VOUT–
VIN–
9
5V
8
7
+
C7
100µF
16V
6
5
CASE
3
DN290 F02
Q1
IRF530
= DIODES INC. B3100
Figure 2. Network Switch Card Monitor with Hot Swap Control
Data Sheet Download
http://www.linear.com/go/dnLTC1921
Linear Technology Corporation
For literature on our Supply Monitors,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2525
dn290f LT/TP 0702 316.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
www.BDTIC.com/Linear
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2002